Smelting of metallic ores has historically been carried out in blast furnaces for so long a period of time (the first recorded use of a blast furnace to produce molten pig iron is in the 14th century A.D.) that both the furnace designs and the basic processes for the making of pig iron are considered to be in the public domain, and recent inventive activity has concentrated more on methods of improved control and optimization to achieve higher yields, higher efficiencies, or better overall furnace economics (for example U.S. Pat. No. 4,273,577 described a control method to optimize product of iron in a given furnace, while U.S. Pat. No. 4,432,790 describes a furnace construction design which minimizes the consumption of coke. U.S. Pat. No. 1,984,793 describes the simultaneous production of Portland cement and pig iron to improve the value of the blast furnace product, while U.S. Pat. Nos. 1,941,793 and 1,964,402 describe methods of reduction of iron ore in blast furnaces which improve the production rates of the furnace). In all of these processes steps are taken to minimize the introduction of fines, since they are generally entrained and lost from the system. In U.S. Pat. No. 4,436,552, methods are discussed of removing the fines from the iron ore charge prior to its being introduced into a blast furnace.
Direct-reduction processes by which the metallic ore is reduced below its melting point, or in some cases reduced above its melting point with a gas rather than with coke or other solid reductant also seek generally to eliminate or minimize the introduction of fines into the reducing apparatus to preserve as high a process efficiency as possible. For example, U.S. Pat. No. 4,238,225 discuses the sintering of iron ore to produce briquets prior to a direct reduction with radio frequency processing, and U.S. Pat. No. 1,849,561 describes the requirement for lump ore for an efficient shaft-furnace process.
U.S. Pat. No. 2,797,989 also describes a process for the production of sponge iron from lump iron ore.
While there are processes and apparati which are designed for the use of finely-divided materials (for example U.S. Pat. No. 2,780,537 describes a method of reducing pulverized iron oxides in a fluidized-bed furnace and U.S. Pat. No. 4,238,226 describes a method of direct reduction of blowing iron oxide powder and pulverized coal into a molten iron bath), none seem to have addressed the problem of exclusively treating the very fine powders which are captured as a result of environmental control. U.S. Pat. No. 4,056,602 discusses the hydrogen reduction of entrained iron oxide in a high-temperature furnace, but does not address the generation of reducing gas nor the separation of complex ores as an integral part of the process.
Environmental control has been responsible for the elimination of much of the visual pollution associated with established industries.
In fact, many of the pejorative descriptions of "smokestack industries" derived from observations of uncontrolled emissions of gas-entrained fines from smelters and kilns, and considerable effort has been spent to keep these emissions to a minimum, thus abating visible atmospheric pollution.
The minimizing of fines emissions in smelting and other processes has produced a number of potential by-product feed materials, both for preparation operations and from capture of the stack fines: for example finely-divided crude iron oxides from taconite beneficiation; "red mud" iron oxide fines from bauxite purification; "breeze" from coal and coke washing and de-dusting, and baghouse dust from electric arc furnace operations. These materials are currently disposed or in simple fashion in ponds or landfills, generally because of the expense required to put them into a form useful for existing technology. The present invention allows the use of these finely-divided materials directly, thus enhancing their economic value and encouraging their recovery rather than environmentally unacceptable disposal.
For the examples of electric arc furnace dust, the dust contains in addition to iron oxides a high concentration of groundwater-leachable zinc and lead oxides which has rendered it no longer disposable in landfill, according to U.S. environmental legislation. Therefore a practical methods to recover the iron, lead, and zinc would eliminate the disposal problem for any slag residue, and would pay for the process through sale of the recovered metals. One method of processing the dust is to agglomerate it with coke into "greenballs" and fire the "greenballs" in a flame-fired rotary kiln (for example, U.S. Pat. No. 2,062,869). This kiln process produces fumes of lead and zinc oxides which are collected in bag filters and then subsequently reduced to the respective metals in a separate reduction step, such as are described in U.S. Pat. Nos. 3,984,235 and 4,017,308. In conventional zinc and lead smelting, the iron and slag from the primary smelting generally are both discarded.
The present invention represents significant improvements over prior art in that no pre-preparation of feed is necessary other than blending of the reductant with the electric arc furnace dust, and all transition metal compounds are beneficially recovered as metals, requiring no further processing steps.